I'm attempting DMA access of a fw_cfg file. I have accessed the directory, compared file names and found the file i want. I have compared the reported size to make sure it is correct for what I was expecting. All of these things are true and then I enter fwcfg_dma_access via

For the posterity of future internet explorers this ended up being an issue in Qemu aarch64. Non-secure devices (FW_CFG at the time of this writing) cannot DMA secure memory (ie Secure ram). Make sure the devices are the same memory security levels.

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